A green body obtained by molding a ceramic powder or a metal powder, or a mixed powder thereof, into a plate form, may be fired and processed after the molding and used as a sputtering target material or an abrasion resistant material. The sputtering target material is used as a material for the preparation of a thin film by a sputtering method and is used for e.g. the production of LCD (liquid display), EL (electroluminescence) or semiconductors. Recently, along with the trend for large sizes of such LCD or semiconductor products, there has been a strong demand for enlarging the target material. Further, with ITO (indium tin oxide) targets, black deposits so-called nodules tend to precipitate on the target surface as the integrated sputtering time increases, which will be a cause for abnormal electrical discharge. It is known that once such abnormal electrical discharge takes place, yield of the product will decrease. Therefore, it is strongly desired to improve the performance of the target material to reduce such abnormal electrical discharge. Accordingly, a large and high density sintered body is strongly desired.
In order to suppress generation of such abnormal electrical discharge and formation of nodules, various studies have heretofore been made e.g. for high densification of target materials or flattening of the sputtering surface. Patent Document 1 discloses that with respect to an ITO target, elements belonging to Groups IIIb and IVb of the Periodic Table such as Al, Si, etc. contained in the target are controlled to be 50 ppm or less, but nothing is specifically mentioned with respect to the influence of carbon. Further, with respect to the influence of carbon in a target, Patent Document 2 discloses that with respect to an ITO target, by an addition of any one of carbon, nitrogen and boron, it is possible to obtain an improvement and stabilization of the etching rate of the obtainable transparent conductive film, and it is preferred that the carbon content is from 0.005 to 3%, the nitrogen content is from 0.1 to 5%, and the boron content is from 0.001 to 10%.
On the other hand, heretofore, as a method for molding a ceramic powder or a metal powder, or a mixed powder thereof, into a plate shape, a dry press molding method, a casting method or a cold isostatic press method has been employed.
The dry press molding method is a method wherein a binder is added to a raw material powder and by means of a mold, a green body is formed, and the casting method is a method wherein a binder is added to a raw material powder to form a slurry, which is cast in a casting mold to produce a cast green body. For the purpose of higher densification, such a green body (primary green body) may further be subjected to cold isostatic pressing to produce a high density post-formed product. As an example, production of an alloy target (IT target) composed of metallic indium and metallic tin, or a composite oxide target (ITO target) composed of indium oxide and tin oxide may be mentioned (e.g. Patent Documents 3 and 4). Such a molding method had a drawback that the production process tended to be complex, although a high density target was obtainable. Further, there was such a drawback that an expensive dry molding die or casting mold was required, and the cost for the molding die tended to be high when a large green body was to be prepared. Further, there was a drawback that the production cost tended to be high since granulation or slurrying of the raw material powder was required.
In contrast with such a production method, Patent Document 5 discloses a method wherein a powder is filled in an inexpensive rubber mold and directly formed under high pressure by cold isostatic pressing without primary molding. Further, with respect to cracking of a green body by restoration of a rubber mold, Patent Document 6 proposes a method for preventing such cracking of the green body by using a rubber having a small rebound resilience value. Further, with respect to cracking of a green body due to fixing of the green body to the rubber mold, Patent Document 7 reports that such cracking can be avoided by incorporating a material which undergoes substantially no elastic recovery at the time of reducing the pressure.
In order to improve the shape accuracy, the above-mentioned Patent Document 5 proposes a method for direct forming under high pressure by cold isostatic pressing by means of a mold with one side made of a metal plate. As a method to obtain a green body having a still better shape accuracy by means of cold isostatic pressing, Patent Document 8 reports a method wherein a powder is sandwiched between a pair of metal plates, vacuum-packed and then preformed by cold isostatic pressing under a pressure of from 5 to 50 MPa, and thereafter, vacuum packing is applied again, and main forming is carried out by cold isostatic pressing. Further, Patent Document 9 proposes a method wherein a mold filled with a raw material powder is closed and sealed, and then inserted in a cold isostatic press, followed by high pressure molding. Patent Document 10 proposes a method for molding by cold isostatic pressing by means of a mold comprising a knockdown mold frame made of a resin and a molding punch. In this method, the mold frame can be disassembled, and it is easy to take out the green body from the mold. Patent Document 11 proposes a molding die comprising a dividable mold frame and a cover made of an elastic material. Patent Document 12 proposes that a molding die comprising a dividable mold frame and a molding punch is covered with a soft material, and CIP molding is carried out.
On the other hand, as the size of a green body becomes large, in order to maintain the green body, its strength is required to be increased. However, in a case where a large green body having a surface of e.g. 3,000 cm2, is to be prepared, it is not possible to apply a large pressure by press molding, and it is not possible to increase the strength of the green body. Therefore, in order to increase the strength, it becomes necessary to add a molding aid such as a binder. Then, a firing step (degreasing step) intended to remove organic substances in the green body by firing will be required. When a molding aid such as a binder is burned or decomposed, the green body is likely to have cracks, and in order to prevent such cracking, it is required to raise the temperature at an extremely low rate, and it will take a long time accordingly. For example, it is disclosed in Patent Document 13 that in order to prepare a plate-form sintered body having a size of 1,000 cm2 or more in good yield, it is necessary to raise the temperature at an extremely low rate of 2° C./hr or less.
Further, in order to increase the efficiency in the use of a target, Patent Document 14 discloses that the erosion region of the target is made thicker than other portions.
Patent Document 1: JP-A-08-246139
Patent Document 2: JP-A-07-187769
Patent Document 3: JP-A-2000-144393
Patent Document 4: JP-A-05-311428
Patent Document 5: JP-A-2003-003257
Patent Document 6: JP-A-09-057495
Patent Document 7: JP-A-06-100903
Patent Document 8: JP-A-09-003636
Patent Document 9: JP-A-05-287315
Patent Document 10: JP-B-08-009120
Patent Document 11: JP-A-61-266204
Patent Document 12: JP-A-2003-266198
Patent Document 13: JP-A-10-330169
Patent Document 14: JP-A-01-290764